So I just bought a PSA CHF 20" upper with 1 in 7 twist. While I was speaking with a few of my buddies yesterday, one of them mentioned that he thought that the 1 in 7 was a little fast for that long of a barrel. I have read multiple threads and the opinions seem to go both ways. I plan on shooting mostly 62gr LC surplus with the occasional case of 55gr commercial thrown in. Should I ditch the FN cold hammer forged 1 in 7 and go to something else or do you all think it will serve my paper punching duties fine like it is? I also think I may handload some of the SMK in the heavier weights for coyotes as well.

I think it is MORE appropriate for 20" than 16", though the barrel length has nothing to do with effect of twist rate. The reason I say that is that the 20" is more suitable for longer range work and heavier/longer bullets are better for longer ranges. Service rifle competitions shoot out to 600 yds and the 75 gr - 90 gr bullets are great for that. And for longer range having a longer barrel the 20" will give slightly more velocity.

I have a PSA upper with 1:7" twist and it shoots everything from 55 gr to 77 gr very well (1/2" - 7/8" 5-shot groups are easy with handloads).

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To be honest I really do not think that I will ever shoot anything under 55gr unless that is all I can find. If that is in fact the case, I would simply shoot it out of one of my 1 in 9 carbines. I have 2 carbines currently and have always wanted a rifle length upper so i decided to take the plunge on a complete upper with BCG and CH. If i shoot anything other than 55gr or 62gr it would be the heavier weights.

I agree that the 1-7" is a great twist for larger projectiles. I have a RRA Varmint 24" barrel AR that has a 1-8" twist. I have yet to use anything that it does not shoot good, but I am a sucker for a fast twist barrel.

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I don't want to hijack the thread, but I believe it's in the same vein as this thread. Is the only problem with a fast twist with lightly constructed bullets of lighter weight? Can you really overspin a bullet in regards to accuracy?

1/7 twist is fine for a 20" barrel, you can stabilize the heaviest bullets in .224 with it. I have heard stories about really lightweight bullets shedding the jacket at these high RPMs, but I shoot 55 grain bullets out of mine @ 3200 fps all the time without problem, mathematically this works out to well over 300,000 rpm (at the muzzle).

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But it does. A shorter barrel produces lower velocity. And to achieve the same level of stability, a slower bullet needs to be spun a little bit faster than the same bullet at a higher velocity.

You are correct that indirectly the barrel length has an influence on bullet RPM by way of greater velocity from a longer barrel (to a point). The difference in velocity between same load in 16" and 20" bbl is at most 200fps compared to say 3,000 fps total or about 7%. Which in practical terms will have no noticeable effect on bullet stability in the normal range of of bullet wright lengths used for 1:7 twist rifles. Perhaps at the margin such as 45 gr bullet the 20" would be slightly better. But I doubt there is any common circumstance in which the 20" bbl is significantly WORSE as implied by the OP's brother.

But I doubt there is any common circumstance in which the 20" bbl is significantly WORSE as implied by the OP's brother.

You're right, it's a pretty small difference and probably not really noticeable in this case. But my only point was that barrel length does affect a round's ideal rifling twist, even if only by a small amount.

Yes. The faster the spin, the more that imperfections in the bullet construction will affect accuracy. Ideally you want to spin it just fast enough to keep it stable, and not too much more.

How big of an effect on accuracy? Probably not enough to matter for most people with most rifles and most ammo. If you are nursing every little bit of accuracy out of your system, then it becomes important.

1:9 is fast enough to stabilize anything that is short enough to fit in an AR magazine. With a barrel under 20" and beyond 500yds you could run into trouble stabilizing bullets 75gr or heavier. Other than that, there's no need to go faster than 1:9.

I've actually run into problems with a 16" 1:9 Bushmaster HBAR stabilizing 75gr and 77gr ammo at 100yds. I was getting 4-5" groups with match ammo. No keyholing but really poor accuracy. At 200yds, I couldn't even keep them on the paper to figure out the group size.

However, I've watched a friend's 16" Armalite 1:9 shoot the same 75gr ammo into a 1" 5-shot group at 100yds.

For me, my general rule is if I know I want to shoot 75gr and up, then I buy a 1:8 or faster twist.

1:9 is fast enough to stabilize anything that is short enough to fit in an AR magazine. With a barrel under 20" and beyond 500yds you could run into trouble stabilizing bullets 75gr or heavier. Other than that, there's no need to go faster than 1:9.

While that may be true down south, it doesn't work that way up here when it gets cold. 1:9 works fine in the summer, but when the temp gets close to zero, 1:9 from a 26" Savage 12FV will keyhole a 75gr HPBT at 50 yards, while 69gr is still accurate. My 1:8 twist AR still stabilizes both of them in the cold.

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One of the factors in the Miller stability formula is air density, and temp and altitude are both a big factor in that.

Also, once a bullet is stable, it will stay that way, even when it gets pushed around as it drops trans-sonic. While forward velocity drops, rotational velocity doesn't, and it works out that the stability factor actually increases as it goes downrange.

I just sold my HK SL8. It has a 20" barrel (510mm) and a 1in7" twist (178mm).
In the manual HK even recommends using 52gr Sierra MatchKings.
Therefore I'm a) sure that a 20" AR15 with a 1in7" twist is not a bad choice and b) that a rather long barrel with a rather short twist will be a good for a wide range of bullet weights, at least from 52 to over 75gr.

I shoot Service Rifle with a 1:7 Colt barrel. It shoots 75gr and 80gr bullets very well. I've used 55gr FMJ bullets for the 200 yard standing stage, and that worked just fine from an accuracy standpoint. I had them to use, so I used them up. Switched to heavier bullets for the 300 and 500 yard stages.

Jimro

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